In lithographic printing, an image area capable of receiving ink and a non-image area capable of repelling ink respectively exist basically almost on the same level on a printing plate, and the difference between both the areas in ink acceptability is used so that the image area only may have ink deposited and subsequently transferred onto a material to be printed, such as paper. The lithographic printing plate usually uses a pre-sensitized plate.
The term "pre-sensitized plate" in this specification means the following.
As described in Teruhiko Yonezawa, "Introduction to Pre-Sensitized Plates (in Japanese)", K.K. Insatsu Gakkai Shuppanbu, Pages 18 through 81, an aluminum substrate treated to be hydrophilic is coated with a lipophilic photosensitive resin layer, and a photo lithographic technique is used to cause the photosensitive layer to leave on the image area and to expose the aluminum substrate surface on the non-image area, for having a dampening water layer formed on the surface to repel ink, for image formation. This plate is called a water-using pre-sensitized plate. There is also a waterless pre-sensitized plate, in which a silicone rubber layer is used as an ink repelling layer instead of the dampening water layer.
The waterless plate means a printing plate, the non-image area of which is made of a material capable of repelling the oil ink usually used for lithographic printing, such as silicone rubber or fluorine-containing compound, for image formation in combination with the image area capable of having ink deposited, to allow printing without using dampening water.
The former water-using pre-sensitized plate is in practice an excellent printing plate, and usually aluminum is used for the substrate. The surface of the aluminum substrate must be able to hold water, and be excellent in adhesiveness to the lipophilic photosensitive resin layer lest the photosensitive layer should peel and come off during printing from the surface. So, the aluminum surface is usually grained, and furthermore, as required, the grained surface is treated by anodized oxidation, etc., to improve the water-holding capacity and the adhesiveness to the photosensitive resin layer. Moreover, to secure the storage stability of the photosensitive resin layer, the aluminum surface is generally chemically treated by zirconium fluoride or sodium silicate, etc.
As can be seen from the above, the process for producing a water-using pre-sensitized plate is complicated and should to be simplified, and yet water-using pre-sensitized plates are widely used since they are excellent in printing capabilities such as printing durability and image reproducibility.
To solve the above problem, new lithographic materials equivalent or superior to the aluminum substrate in printing capabilities, but lower in material cost and simpler to produce are proposed as substitutes for the aluminum substrate. For example, Japanese Patent Publication (Kokoku) No. 56-2938 proposes to use a substrate coated with an ink repelling layer composed of a hydrophilic high polymer, instead of the aluminum substrate, and to form a photosensitive layer on the substrate. However, since a water resistant layer composed of an aldehyde condensation product of polyvinyl chloride, polyurethane or polyvinyl alcohol is simply covered with a hydrophilic layer composed of urea resin, the hydrophilic layer is insufficient in ink repellency, and in addition, poor in adhesion to the photosensitive resin layer, resulting in its being insufficient in printing durability. Furthermore, Japanese Patent Laid-Open (Kokai) No. 57-179852 proposes to coat a substrate with a hydrophilic radical-polymerized compound, and to irradiate the surface of the substrate with an active beam for making it hydrophilic, and then to cover it with a photosensitive resin layer. However, the hydrophilic surface layer formed by this method is rigid and insufficient in ink repellency, and also poor in printing durability.
Moreover, since the development of these water-using pre-sensitized plates dissolves the photosensitive layer for exposing the surface of the aluminum substrate, the photosensitive layer is inevitably dissolved into the developer, and the developer is significantly changed in chemical composition, to thus becoming "fatigued" in a short time. So waste developer is generated in a large quantity.
Thus, the developer must be exchanged frequently for maintenance. Furthermore, the waste developer generated must be treated with enormous efforts and cost.
On the other hand, as a simple version of water-using pre-sensitized plates, directly image-forming lithographic original plates are widely practically used. In these plates, an image is formed in an image receiving layer composed of a toner, etc. formed on a substrate such as paper, using a plain paper copier, and the non-image area is desensitized by an etchant, etc., to convert the image receiving layer into an ink repellent layer. Concretely, generally an image receiving layer composed of a water soluble binder polymer, inorganic pigment, water resisting reagent, etc. is formed on a water resistant substrate. Techniques of this category are proposed in U.S. Pat. No. 2,532,865, Japanese Patent Publication (Kokoku) No. 40-23581, Japanese Patent Laid-Open (Kokai) Nos. 48-9802, 57-205196, 60-2309, 57-1791, 57-15998, 57-96900, 57-205196, 63-166590, 63-166591, 63-317388, 1-114488, 4-367868, etc. For these direct image-forming lithographic original plates, the image receiving layer proposed to be converted into an ink repelling layer is composed of a water soluble polymer hydrophilic already before etching, such as PVA, starch, hydroxyethyl cellulose, casein, gelatin, polyvinyl pyrrolidone, vinyl acetate-crotonic acid copolymer, or styrene-maleic acid copolymer, or a water dispersible polymer such as an acrylic resin emulsion, and an inorganic pigment such as silica or calcium carbonate, and also a water resisting reagent such as melamine formaldehyde resin condensation product. Japanese Patent Laid-Open No. 63-256493, etc. propose a direct image-forming lithographic original plate which uses a hydrophobic polymer to be hydrolyzed by etching for generating hydrophilic groups, as a main component.
These direct image-forming lithographic original plates inevitably require etching for converting the image receiving layer into an ink repelling layer, and without etching, they little show ink repellency.
To obtain practical ink repellency, etching and the use of a hydrophilic binder polymer in a large quantity are required, but these tend to degrade water resistance, lowering printing durability. Furthermore, enhancing the hydrophilicity involves a problem that the adhesiveness to the image formed by a toner, etc. tends to decline. On the other hand, if the amount of the water resisting reagent added is increased or a hydrophobic polymer is added, respectively to enhance water resistance for improving printing durability, there arises a problem that the hydrophilicity is lowered to considerably lower the ink repellency.
Printing plate techniques-based on one processing step of exposure only without requiring development, lacquering and etching at all, by applying the hydrophilicity/hydrophobicity conversion reaction developed by Union Carbide are disclosed in Japanese Patent Publication (Kokoku) Nos. 42-131, 42-5365, 42-14328, and 42-20127, U.S. Pat. Nos. 3,231,377, 3,231,381, 3,231,382, etc. The plates are coated with an association product of polyethylene oxide and phenol resin, together with a photosensitive material. However, these plates are not practical enough, since they are rigid and poorly flexible in the non-image area and insufficient in ink repellency, and since the difference between the ink repellency of the non-image area and the ink anchoring capability of the image area is too small.
Furthermore, in the case of water-using pre-sensitized plates, the quantity of dampening water must be constantly controlled, and properly controlling the quantity of dampening water requires substantial techniques and experience. Moreover, the isopropanol added to dampening water as an essential ingredient tends to be severely regulated in recent years in view of occupational health environment and waste water treatment, and this must be urgently responded to.
On the other hand, waterless pre-sensitized plates using a silicone rubber layer as the ink repellent layer instead of dampening water are being rapidly widely adopted in recent years and are highly practical, since the dampening water control work required for the water using pre-sensitized plates is not required at all to allow very simple printing work, as disclosed in Japanese Patent Publication (Kokoku) 54-26923, 57-3060, 56-12862, 56-23150, 56-30856, 60-60051, 61-54220, 61-54222, 61-54223, 61-616, 63-23544, 2-25498, 3-56622, 4-28098, and 5-1934, and Japanese Patent Laid-Open (Kokai) Nos. 2-63050 and 2-63051, etc. However, since a silicone rubber layer weak in mechanical strength is used as the ink repellent layer, its durability is insufficient, and an ink repellent material excellent in durability is highly demanded.
Furthermore, in the development, the silicone rubber layer must be mechanically removed by brushing, and the waste developer containing the removed silicone rubber particles is generated in a large quantity. In addition, the life of the brush is short, to require the frequent exchange of brush, and a maintenance action must be taken to collect and dump the silicone rubber particles.